Synthetic resins having melting temperatures of 300° C. and above exhibit good heat resistance, mechanical strength, mechanical rigidity, chemical resistance, flame retardance, processability on a molding or extruding machine and the like, and have conventionally achieved wide use for many applications such as in the production of automobile, mechanical, electrical and electronic parts.
However, the recent remarkable advances in technology require these synthetic resins to achieve further property improvements, such as in heat resistance, mechanical strength and rigidity, while maintaining the desirable properties they intrinsically possess.
The loading of fibrous inorganic fillers, such as glass fibers, carbon fibers, wollastonite and potassium titanate fibers, in synthetic resins is known to considerably improve mechanical strength, rigidity, heat resistance and other properties thereof. Resin compositions loaded with such glass fibers or carbon fibers however provide an extremely roughened surface and a poor appearance. Also, resin compositions loaded with wollastonite or potassium titanate fibers are highly anisotropic and in some cases yield variations in linear expansion coefficient, among mechanical properties.
The loading of powder- or flake-like inorganic fillers such as calcium carbonate, mica and talc, while sufficient to lower a mold or die shrinkage factor, a linear expansion coefficient or the like and thus increase the dimensional stability, is insufficient to achieve improvements in mechanical strength and heat resistance. Also, the loading of such inorganic fillers in synthetic resins having melting temperatures of 300° C. and above causes hydrolysis of such synthetic resins, which is considered due to either alkaline contents (e.g., sodium and potassium) liberated from the inorganic fillers, or interlayer water from the inorganic fillers, or weakly acidic or alkaline nature of the inorganic fillers. This is also accompanied by the molecular weight reduction which leads to a drop of molding stability and a loss of desirable properties intrinsic to such synthetic resins, which have been problems.
Synthetic resins having melting temperatures of 300° C. and above are also being used for heat-resistant films for use in flexible printed circuit boards. A polyimide resin is representative of materials useful for such heat-resistant films. However, as the technology continues to push up density and integration levels of circuits, the polyimide resin because of its high hygroscopicity will likely become insufficient to provide circuit reliability, which will be a problem. In addition to be high in price, the polyimide resin film can not be laminated onto a metal foil without the use of an adhesive. These problematically increase a total cost.
The above-described polyimide resin is a thermosetting polyimide resin. The use of a thermoplastic resin, such as a thermoplastic polyimide resin, is now under investigation as a possible alternative. The use of the thermoplastic polyimide resin or other thermoplastic resins enables recycling and reduction of a total cost since they can be laminated onto a metal foil by a film extrusion technique. However, the sole use of the thermoplastic polyimide resin results in the insufficient mechanical strength and heat resistance. Also because the thermoplastic polyimide resin has a high linear expansion coefficient in the range of 4-5×10−5° C.−1, curling inevitably occurs when it is laminated onto a metal foil having a linear expansion coefficient of 1-2×10−5° C.−1, which has been a problem. That is, the film is adhered to the metal foil in the laminating process. In the case where the film differs largely in linear expansion coefficient from the metal foil, a resulting laminate of the film and metal foil when cooled to ambient temperature is curled due to the dimensional difference between the top and bottom.
Attempts have been made to improve mechanical strength, heat resistance or the like of thermoplastic resins such as a thermoplastic polyimide resin or to reduce their linear expansion coefficients by loading inorganic fillers therein. Examples of proposed inorganic fillers include powder-form inorganic fillers such as mica, talc and silica; inorganic fibers such as potassium titanate fibers; and the like.
However, the loading of such inorganic fillers in thermoplastic resins results in the production of rigid, less flexible and thus very brittle films. Another problem is the failure to impart the contemplated linear expansion coefficients to resulting films. Other problems arise when the above-described hydrolysis of resins is caused to take place by the alkali contents liberated from inorganic fillers. That is, the resulting molecular weight reduction of resins lowers desirable properties intrinsic to such resins and results in the difficulty to take a film off.